Skip to main content

Full text of "The Cambridge Natural History"

See other formats

28                               QUADRATE   AND   INCUS                           CHAP.
matter. But the hall-mark of truth is not always simplicity;
indeed the converse appears to be frequently the case. And
on the whole this view does not commend itself to zoologists
at present. For it must be borne in mind that the lower jaw of
the mammal is not the precise equivalent of that of the reptiles.
Apart from the membrane bones, which may be collectively the
equivalents of the dentary of the mammal, there is the cartilaginous
articular bone to be considered, which forms the connexion
between the rest of the jaw and the quadrate in reptiles. Even
in the Anomodontia, whose relations to the Mammalia are con-
sidered elsewhere, there is this bone. But in these reptiles the
articular bone articulates not only with the quadrate, but also to
a large extent with the squamosal, the quadrate shrinking in
size and developing processes which give. to it very much the
look of either the incus or the malleus of the mammalian ear.
In fact it seems on the whole to fit in with the views of the
majority, as well as with a fair interpretation of the facts of
embryology, to consider that the chain of ear bones in the
mammal is not the equivalent of the columella of the reptile,
but that the stapes of the mammal is the columella, and that
the articulare is represented by the malleus and the quadrate
by the incus. It is very interesting to note this entire change
of function in the bones in question. Bones which in the reptile
serve as a means of attachment of the lower jaw to the skull are
used in the mammal to convey the waves of sound from the
tympanum of the ear to the internal organ of hearing.
Another important and diagnostic feature in the mammalian
skull is that the first vertebra of the vertebral column always
articulates with two separate occipital condyles, which are borne
by the exoccipital bones and formed mainly though not entirely
"by them. Certain Anomodontia form the nearest approach to
the mammals in this particular. The two condyles of Amphibia
are purely exoecipital in origin.
In the Mammalia, unlike what is found in low&r "Vertebrates
(but here again the Anomodontia form, at least a partial exception),
the jugal arch does not connect the face with the quadrate, for,
as already said, that bone does not exist, in the Sauropsidan
form, in mammals. This arch passes from the squamosal to the
maxillary, and has "but one separate bone in addition to those
two, viz. the jugal or malar.